Earthquakes, volcanoes, mountain building, ice ages, landslides, floods, life evolution, plate motions—all of these phenomena have interacted over the vast expanses of deep time to sculpt the dynamic planet that we live on today. Planet Earth presents an overview of several aspects of our home, from a geological perspective. We begin with earthquakes—what they are, what causes them, what effects they have, and what we can do about them. We will emphasize that plate tectonics—the grand unifying theory of geology—explains how the map of our planet's surface has changed radically over geologic time, and why present-day geologic activity—including a variety of devastating natural disasters such as earthquakes—occur where they do. We consider volcanoes, types of eruptions, and typical rocks found there. Finally, we will delve into the processes that produce the energy and mineral resources that modern society depends on, to help understand the context of the environment and sustainability challenges that we will face in the future.

Avis

RC

Great course, despite the glitches in the links, etc., which seemed to be outdated. i really gained little from the forum.

KS

Nov 15, 2017

Filled StarFilled StarFilled StarFilled StarFilled Star

Excellent course! Very informative without being tedious. Clear and concise delivery and illustrations. Would recommend!

À partir de la leçon

Week 2: Plate Tectonics

In the early twentieth century, publication of the hypothesis on continental drift caused an uproar that soon died down. Data collected in mid-century led geologists to reconsider the idea that continents could move. During the 1960s and 1970s, old ideas were reworked into what is now called the theory of plate tectonics. As we will see, this robust theory encompasses many geological phenomena that appear to be unrelated at first glance: earthquakes and volcanoes, but also ice ages, fossils, and mountains. Today, plate tectonics provides an overarching framework for interpreting the Earth. We study its details in Week 2, but we will return to this theory again and again throughout the rest of this course.

Enseigné par

Dr. Stephen Marshak

Professor and Director of the School of Earth, Society, and Environment

Dr. Eileen Herrstrom

Lecturer

Transcription

[SOUND] [MUSIC] So if the glaciations effected the southern portion of Pangea then the rest of Pangea must have extended into more equatorial climates. And in fact when he drew a map of Pangea on a globe he found that portions of Pangea extended across the equator and into the northern hemisphere. Well if you look at a map of climate belts today, it's clear that equatorial belts tend to be tropical, like rain forests, and a lot of rain, a lot of vegetation, very warm oceans. That on either side of the equatorial belts, there tend to be deserts in subtropical regions. And then furhter north and south of that, you enter temperate regions. So Wagner drew climate belts on his map of Pangaea and then compared them to what was known at the time about the distribution of rock types whose character was indicative of particular climate regimes. So he looked at distribution of distinctive sedimentary deposits that could be correlated with particular climate belts and placed those sedimentary deposits on his map of Pangea. So coal deposits indicative of places where there was lots of vegetation lots of rainfall, probably warm climates. Therefore indicative of tropical regimes occurred in what turned out to be the equatorial regions of his map of Pangea. Similarly, reefs, deposits of shells that formed typically in warm water, also occurred in the belt of Pangea that would have been tropical. In the regional either side, were one would expect to have dessert regions, just like the Sahara for example occurs to the north of the tropical regions of Africa. He found that there were indeed deposits of sandstones that have characteristics indicative that they were once desert dunes. And they also found thick accumulations of salt deposits, again indicative of a subtropical environment. So, the distribution of these deposits made no sense on the modern map of the world but they did make sense on Wagener's map of Pangea. So we considered this to be also evidence in support of the model. By the time that he was doing his work there was a fair amount of information about the distribution of fossil species on the planet. And paleontologists, people who study fossils, realized that there were some fossils that were indicative of marine organisms, and there were some fossils that were indicative of land organisms. Now, if you look at the world today, it's clear that there are certain kinds of land organisms that only occur on certain continents and not on others. Because they can't swim, they can't get across the vast expanses of ocean that separate continents. For example there are kangaroos on Australia, but there are no kangaroos in North America even in comparable climates. So Wagner had this idea in mind and looked at the distribution of land organisms. For example, species of amphibians, species of plants, and other kinds of organisms, that could only have grown on land and looked at where they were distributed. Once again, they were occurring on all continents, really easy if you could walk from one continent to another, virtually impossible if you had to cross an ocean. So, again he took the distribution of fossil species, one more bit of evidence that suggest his reconstruction of Pangia made sense. Finally Wagner decided to look into distribution of distinctive packages of rock. Now at this point in this course it's probably a kind of vague statement to say packages of rock. But basically there are assemblages of kinds of rock, kinds of structures in rock, that are distinctive. So geologist can recognize and identify an association of rocks just like an art critic can recognize one artist painting from another artist's painting. Or an architect can recognize one kind of architecture from another kind of architecture. Well, to make a long story short, Wegner was able to recognize distinctive assemblages of rock that occurred either side of the Atlantic Ocean. And when he closed up the Atlantic Ocean and put Pangea back together, these packages of rock were immediately adjacent to each other as if they were once together and later split apart. Similarly when he looked the mountain belts that occurred on either of the Atlantic, he found that there was a mountain belt on the East side of North America. There were mountain belts on the West side of Europe,they matched when they were put together. So again Wegner concluded that the distribution of distinctive rock units was compatible with his reconstruction of Pangaea. Notice I haven't said that anything is proven, the existence of Pangea, I'm simply saying that Wegner recognized that these observations were compatible with the idea of Pangea. Well, Wegener's ideas were so radical, because what he was basically saying was that the fixist concept, or the fixist view of the planet was wrong and that in fact continents did move over time. That idea was so radical that it just didn't sit well with the rest of the geological community. There were a number of meetings, perhaps one of the most famous of which took place in 1926. Wegener tried to support his concept of the Pangea, but he was basically attacked by the bulk of the geologic community. The main complaint was that Wegener could not come up with an explanation as to why the continents moved, or how the continents moved, or what drove them. He had some ideas, but the other geologists were able to show that these ideas couldn't possibly work. For example, Wegener somehow envisioned that the continents somehow plowed through the ocean floor as a ship would plow through oceanic water. But that's impossible because the rock of the ocean floor is too strong and other geologists knew that. Wegener knew that himself but basically couldn't come up with an alternative. Similarly, Wegener tried to understand what could possibly cause Pangea to break apart. The only thing he could come up with was the idea that it was centrifugal force, the spin of the Earth. But again, calculations showed that's impossible, there simply isn't a strong enough force to move masses the size of continents. [MUSIC]